Can someone explain exactly what a Lambda sensor does and how it works. Also whats the difference between open and closed loop.

 

The purpose of the lambda/oxygen sensor in the exhaust system is to report the amount of oxygen in the exhaust. The ECU needs to know this primarily to minimise emissions. A chemical reaction in the sensor creates a voltage that varies with the comparative rich-ness or lean-ness of the mixture.

The voltage produced by the lambda sensor is read by the ECU, and the ECU then uses this data to calculate the correct amount of fuel to inject when the car is running in closed loop mode.

So what's the difference between closed loop and open loop mode?

For minimal emissions, and optimal functioning of the catalytic converter fitted to all modern cars, the ideal air to fuel ratio is 14.75:1 (the so-called stoichiometric point).

In closed loop mode, the ECU tries to maintain the lambda sensor's voltage at a level that corresponds to stoichiometry. In other words, if the lambda voltage is a little lower than ideal, it will add more fuel to the next injection cycle. If the lambda voltage is higher than ideal, it will remove a bit of fuel from the next cycle, and so-on.

If the boost level or engine speed rises above preset levels, a richer mixture than stoich is necessary for safe and proper functioning of the engine, so the ECU will transition to open loop mode to set the mixture.

This means that it will ignore the lambda sensor input, and calculate the amount of fuel it injects based on a set of tables (or maps) in conjunction with data it receives from the mass airflow sensor in the inlet tract, manifold absolute pressure (i.e. boost pressure sensor), engine speed and so-on.

[Edited by greasemonkey - 1/21/2004 6:44:43 PM]

 

Thanks for that greasemonkey.